With the rapid development of information technology, electrical document are widely used. However, for the purpose of storage, processing and delivery via internet network of the documents, it is necessary to scan the paper document into the format of electrical documents. Currently, the scanner is supplied with the power from the universal serial bus (USB) of the computer or with the proper voltage level which is generated by the conversion of the mains supply via an external adaptor. These two kinds of powers supply are determined while the scanner is designed, which is in an invariable status. Moreover, when the scanner is electrically connected to the computer for use, the power supply mode of the scanner is identified by the computer. However, the above-mentioned ways have restriction and inconvenience. When the scanner is supplied with the power from USB signal, the power supply ability of USB is lower, resulting in decreasing the operation speed of the scanner. When the scanner is supplied with the proper voltage level which is generated by the conversion of the mains supply via an external adaptor, the portability of the scanner is downgraded and it is necessary to find the power socket of the mains supply disadvantageously. Consequently, there is a need to improve the conventional scanner to solve the aforementioned problems.